Epidemiology Notes 2012-10-02

In the News

Calculations

The Great Epidemic

Epiville SARS outbreak

Walk through the exercise and have students answer the question.

Why Influenza?

Influenza kills more people in the US than any other infectious disease, including HIV. This involves a combination of transmissibility, ability to evade the immune system, and therefore potential lethality.

Why is the influenza virus so successful. * Structure of viruses and their rapid mutability. They don’t have to do anything much on their own, so they can afford to mutate, and still get replicated. * Two main components that interact with receptors on the cell surface: * hemagglutinin which binds to sialic acid receptor * neuraminidase which breaks up the sialic acid remaining on the cell surface to keep the escaping viruses from binding to the cell. H1N1 coding scheme. * RNA leads to rapid mutation. * Reassortment — since there are several different gene segments in the virus — leads to the possibility of mixing genes from different viruses. Pigs have sialic acid receptors that can bind to both bird viruses and mammalian viruses, providing a means for reassortment. (Some claim that human cells can provide a means for combining, too.) * p 105-6 “The influenza virus mutates so fast that 99 percent of the 100,000 to 1 milion new viruses that burst out of a cell in the reproduction process are to defective to infect another cell and reproduce again. But that still leaves between 1000 and 10,000 viruses that can infect another cell.” — What’s the reproduction time: around 10 hours. Trace through the process over 2 days — what proportion of the body’s cells can get infected.

Immune response

  • Capillaries must dilate to admit white blood cells. How do you get them to dilate? Fever?
    • Red blood cells are as big as a capillary: “The size of erythrocytes varies widely among vertebrate species; erythrocyte width is on average about 25% larger than capillary diameter and it has been hypothesized that this improves the oxygen transfer from erythrocytes to tissues.” “A typical human erythrocyte has a disk diameter of approximately 6.2-8.2 µm[27] and a thickness at the thickest point of 2-2.5 µm and a minimum thickness in the centre of 0.8-1 µm, being much smaller than most other human cells.”
    • White blood cell diameter, approx. 10 µm: see table Macrophages are 5 times larger.
  • Immune response to one influenza virus recalls the immune response to other varieties to which the body has previously been exposed. Thus, a new virus tends to suppress the spread of old viruses. There tends to be a dominant form of virus in peak seasons and it’s usually a relatively new form, to which many people have not yet been exposed.

Population response

  • p. 114 Influenza pandemics generally infect from 15 to 40 percent of a population. Why not more?

Disease

Classic epidemiological problem: account for an outbreak.

Basic tools: * accounting for exposure and risk * time and space pattern of outbreak. We’re going to be focusing in this section particularly on the time pattern of an outbreak.

Epidemic Curves

  • Epidemic curve for a point-source epidemic. EE Figure 12.4 point-source
  • Epidemic curve for a propagative epidemic, in the early phases when the waves are still distinct. EE Figure 12.5
  • The Danish tuberculosis epidemic. EE Figure 12.6
  • Conversion rate for students in different settings: EE Table 12.2
  • Long-term tracking of cases EE Table 12.3

Discussion of P-CAST Report

The report.

  • Look at their scenario in Box 3A on page 19. What are the assumptions that are going into it? (Ten-fold increase every week, about half the population susceptible. Sketch out the incidence, and the cumulative incidence curves.)
  • Page 2: “In Australia, 11 percent of over 20,000 confirmed cases of 2009-H1N1 influenza have been hospitalized. And of the 410 cases now hospitalized, 110 are in ICUs.” Why would it be misleading to say that 1/4 of hospitalized cases go to ICU.
  • Why was the initial case-fatality rate estimated in Mexico so high?
  • Note how they used an estimate to discover a bottle-neck — anticipating 50-100% of ICU capacity will be needed. But no estimate of how long people will stay in the ICU.
  • What’s the problem with a single go/no-go decision, which they identified as the problem in the 1975 swine-flu situtation?

Simulation of Infection Spread

Hand out the transmission tokens * Students should separate the infection tokens and place them face down.
* I’ll infect one student. He or she should turn over a random token and infect that student. That student passes along the infection in a similar way. * Repeat, but with each infective student using two tokens. * Repeat, but with three. * Herd immunity: three tokens but half the class is immune (as described by the immune/susceptible tokens)

Distribution of Diseases in Different Countries

EE Figure 12.1
A horrible, side-by-side pie chart graph. Talk about how to make the graphic more effective. The key principle is to bring adjacent the things that you want to compare.

Outbreaks

Different types of outbreaks: * point source, limited time (food poisoning) * point source, environmental toxin (uranium mine tailings) — might show an increase as more people are exposed for more time * transmissible: endemic, epidemic, pandemic (enzootic, epizootic)

Basic Information about Disease

  • Examples of diseases and conditions: Acute/Chronic, Infectious/Noninfectious, Communicable/Noncommunciable from MT p. 47
  • Inanimate sources: inanimate sources
  • Portal of entry and exit: portals
  • Infectious disease names and classification Friis and Sellers p. 440
  • Bacterial Microbe Sources from MT p. 52
  • Other Microbe Sources from MT p. 52 *Available vaccines from MT p. 62 from MT p. 62

Spread and Immunization

  • Severity severity
  • Sources of resistance resistance

Herd Immunity

Diagram of a population, showing a low immunization level which falls short of protecting individuals within the group. From MT p. 64 and effective immunization effective immunization

*Levels of Preventive Measures